Getting object's parent namespace in python?

Question

In python it's possible to use '.' in order to access object's dictionary items. For example:

class test( object ) :
  def __init__( self ) :
    self.b = 1
  def foo( self ) :
    pass
obj = test()
a = obj.foo

From above example, having 'a' object, is it possible to get from it reference to 'obj' that is a parent namespace for 'foo' method assigned? For example, to change obj.b into 2?

Answer 1

Python 2.6+ (including Python 3)

You can use the __self__ property of a bound method to access the instance that the method is bound to.

>> a.__self__
<__main__.test object at 0x782d0>
>> a.__self__.b = 2
>> obj.b
2

Python 2.2+ (Python 2.x only)

You can also use the im_self property, but this is not forward compatible with Python 3.

>> a.im_self
<__main__.test object at 0x782d0>

Answer 2

On bound methods, you can use three special read-only parameters:

• im_func which returns the (unbound) function object
• im_self which returns the object the function is bound to (class instance)
• im_class which returns the class of im_self

Testing around:

class Test(object):
    def foo(self):
        pass

instance = Test()
instance.foo          # <bound method Test.foo of <__main__.Test object at 0x1>>
instance.foo.im_func  # <function foo at 0x2>
instance.foo.im_self  # <__main__.Test object at 0x1>
instance.foo.im_class # <__main__.Test class at 0x3>

# A few remarks
instance.foo.im_self.__class__ == instance.foo.im_class # True
instance.foo.__name__ == instance.foo.im_func.__name__  # True
instance.foo.__doc__ == instance.foo.im_func.__doc__    # True

# Now, note this:
Test.foo.im_func != Test.foo # unbound method vs function
Test.foo.im_self is None

# Let's play with classmethods
class Extend(Test):
    @classmethod
    def bar(cls): 
        pass

extended = Extend()

# Be careful! Because it's a class method, the class is returned, not the instance
extended.bar.im_self # <__main__.Extend class at ...>

There is an interesting thing to note here, that gives you a hint on how the methods are being called:

class Hint(object):
    def foo(self, *args, **kwargs):
        pass

    @classmethod
    def bar(cls, *args, **kwargs):
        pass

instance = Hint()

# this will work with both class methods and instance methods:
for name in ['foo', 'bar']:
    method = instance.__getattribute__(name)
    # call the method
    method.im_func(method.im_self, 1, 2, 3, fruit='banana')

Basically, im_self attribute of a bound method changes, to allow using it as the first parameter when calling im_func

Answer 3

since python2.6 synonyms for im_self and im_func are __self__ and __func__, respectively. im* attributes are completely gone in py3k. so you would need to change it to:

>> a.__self__
<__main__.test object at 0xb7b7d9ac>
>> a.__self__.b = 2
>> obj.b
2